Jac Caglianone, Kristian Campbell, and Marcelo Mayer - Fantasy Baseball Prospects Risers, Fallers

June 7, 2025

Dawn analyzes top fantasy baseball prospects risers, fallers, sleepers, and busts for 2025. Her MLB rookies report for Week 11 and recently promoted prospects.

Welcome to the newest edition of RotoBaller’s Rookie Report -- your trusted source for tracking the progress of MLB’s most exciting up-and-comers. In this issue, we’re focusing on three players who’ve recently made the leap to the majors or retain rookie status heading into the 2025 season.

Last time, we took a closer look at Matt Shaw, Luke Keaschall, and Agustin Ramirez -- each showing glimpses of potential as they navigated the early challenges of professional baseball.

This week, the spotlight turns to Jac Caglianone, Kristian Campbell, and Marcelo Mayer. We'll examine each's performance and explore what their development could mean for their big league teams moving forward.

Jac Caglianone, 1B/OF, Kansas City Royals

Caglianone finally made his MLB debut on June 3 at Busch Stadium against the Cardinals. The Royals' top prospect has been called up at a critical time for Kansas City, as it needs more offensive production to support its excellent pitching staff.

Track Record:

Out of the University of Florida, the sixth overall pick by K.C. in the 2024 MLB Draft started as a two-way player before transitioning to hitting full-time. In his first full professional season, the 6-foot-5, 250-pound lefty had his way with minor league pitchers, slashing .322/.389/.593 with 15 homers and 56 RBI.

Outlook:

Kansas City ranks dead last in MLB with only 34 home runs and is averaging 3.3 runs per game despite having the second-best pitching staff in baseball. Caglianone's max exit velocity of 113.6 mph puts him in the same class as established sluggers like Matt Olson, Austin Riley, and Cal Raleigh. Despite his elite power, he has a 20% strikeout rate.

The first Major League hit and RBI for Jac Caglianone! pic.twitter.com/bbUgc0OP6M

— MLB (@MLB) June 5, 2025

Caglianone's chase rate has caused concern as well as whether or not big league pitching might exploit his aggressive approach, but Kansas City's offensive struggles have forced its hand. With the team sitting at 31-29 and needing a spark, Caglianone gives it an immediate chance to break through its string of cold bats.

Kristian Campbell, 2B, Boston Red Sox

The Red Sox's breakout prospect started the 2025 season on fire, earning AL Rookie of the Month honors for March/April before signing a $60 million, eight-year contract extension. However, recent struggles have raised questions about rookie adjustments and whether he needs a reset.

Track Record:

The 2023 fourth-round pick out of Georgia Tech went undrafted out of high school and was even redshirted as a freshman before everything came together. After signing for just $492,700, Campbell added 15 pounds of muscle and completely revamped his swing.

The results spoke for themselves -- he led the minor leagues with a 180 wRC+ in 2024 while slashing .330/.439/.558 with 20 homers and 24 swiped bags. He had only 19 games of Triple-A experience before making Boston's Opening Day roster.

Outlook:

Campbell's rookie season of ups and downs displays the challenges young players often face. After slashing .301/.407/.495 with four homers in March and April, May's results haven't been as impressive. Since May 1st, he has a .137 AVG (13-for-95) with just one homer and a .383 OPS across 26 games.

Sorry if it offends (and I know it will trigger Red Sox fans that have their favorites).

Kristian Campbell has not been an MLB caliber player for more than a month and a half. He’s a rookie. This happens. I still think he’s going to be great. But he may need a reset of some… pic.twitter.com/2FaVhqU1df

— Ryan Martin (@remartin_34) June 4, 2025

His struggles and Boston's disappointing 30-34 start have created new pressures for team management. While manager Alex Cora insists he wants Campbell to work it out at the major league level, the reality (and Red Sox fans on X) is harder to ignore. Campbell may have been rushed to the majors, and a reset in Worcester could provide the development time he missed last year.

Marcello Mayer, SS/2B/3B, Boston Red Sox

The fourth overall pick in the 2021 draft, Mayer finally got his long-awaited call to the majors on May 24th when Alex Bregman was placed on the IL with a quad strain. The highly touted shortstop made his debut at third base and, after going hitless in his first game, recorded his first MLB hit and an extra-base hit the following day.

Track Record:

Mayer has been one of the most closely watched prospects in baseball since the Red Sox made him the highest draft pick in franchise history, paying a then-record $6.66 million signing bonus. The lefty from Eastlake High School in California was considered the best overall prospect in the 2021 draft.

After working his way through the system, he reached Triple-A Worcester this season and slashed .271/.347/.471 with nine homers and 43 RBI across 43 games.

Outlook:

Mayer's promotion comes at an interesting time for Boston's infield setup. With Campbell struggling and the Red Sox needing to fill holes created by injuries, Mayer's versatility makes him valuable. He's been getting extensive work at second base in Triple-A, suggesting the organization sees him potentially taking over that position if Campbell moves to first base or continues to struggle.

Alright, that settles it… Marcello Mayer all-time Red Sox third baseman pic.twitter.com/i9RJS2LvI2

— Jordan Moore (@iJordanMoore) May 31, 2025

The 22-year-old's debut was a typical rookie experience -- three strikeouts in his first game before settling in with his first hit the next day. Mayer's baseball IQ should help him adjust to the major league level more quickly.

As Boston's No. 2 prospect and No. 8 overall in MLB, expectations are high, but his track record suggests he has the tools and makeup to succeed. The Red Sox are hoping he can provide the steady presence their infield needs while continuing his development at the highest level.

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